1. Field of the Invention
This invention relates to a coin type (button type) non-aqueous electrolyte secondary battery using a material capable of storing and emitting lithium as an active material for both negative and positive electrodes, and a lithium ion-conductive non-aqueous electrolyte.
2. Background of the Invention
The coil type (button type) non-aqueous electrolyte secondary battery according to the prior art has the features that it has a high energy density and is light in weight. Because of such features, the application of the coin type non-aqueous electrolyte secondary battery to a back-up power source for appliances has been increasing.
Almost all the conventional coil type (button type) non-aqueous electrolyte secondary batteries have to add lithium in any form to an active material of a negative electrode. In the case of a battery using a lithium-aluminum alloy for the negative electrode and a 3 V-class lithium-containing manganese oxide for the positive electrode, for example, it has been necessary to press-bond lithium to aluminum of the negative electrode. In the case of a battery using carbon for the negative electrode and the 3 V-class lithium-containing manganese oxide for the positive electrode, it has been necessary to electrochemically introduce lithium to the negative electrode.
In the batteries of the kind described above, the material of a gasket for keeping air- and gas-tightness of the battery and insulation between the positive and negative electrodes is of utmost importance. Economical polypropylene has been used in the past as the material of the gasket because it is excellent in chemical resistance, flexibility and creep resistance, has high moldability and can be injection-molded.
When the battery is used mainly as the memory back-up power source, the battery is soldered in most cases to a printed substrate together with a memory device after terminals for soldering are welded to the battery. This soldering to the printed substrate has been conducted conventionally by using a soldering iron. However, as a greater number of electronic components are required to be mounted inside the same area of the printed substrate with scale-down or higher functions of appliances, it has become more and more difficult to secure the space into which the soldering iron is inserted. Automation of the soldering work itself has been required in order to reduce the cost of the soldering work.
Therefore, a method has been developed which applies in advance a solder cream, or the like, to soldered portions on the printed substrate, mounts the components onto such portions or supplies a small solder ball to each portion after the components are mounted, and passes the printed substrate assembly having the components mounted thereto through a furnace of a high temperature atmosphere set to 200 to 300xc2x0 C., for example, so that the soldered portions reach a temperature higher than the melting point of the solder so as to fuse the solder and to thereby execute soldering (hereinafter called xe2x80x9creflow solderingxe2x80x9d). The coin type (button type) non-aqueous electrolyte secondary battery having the conventional construction involves the drawback that the function of the battery is lost at the time of reflow soldering because the battery does not use the materials in consideration of the heat resistance.
Because almost all the conventional coin type (button type) non-aqueous electrolyte secondary batteries need to add lithium in any form to the active materials of the negative and positive electrodes during the production process as described above, the production method has to use the lithium metal which is difficult to handle.
When lithium is added in any form to the active materials of the negative and positive electrodes during the production process, the secondary battery lacks stability in reflow soldering.
In a coin type (button type) non-aqueous electrolyte secondary battery using a 3 V-class lithium-containing manganese oxide Li4Mn5O12 for the positive electrode and an lithium-aluminum alloy for the negative electrode, for example, the electrolyte and the lithium alloy react with each other during reflow soldering in almost all combinations of the electrolytes and heat-resistant battery members, and invite drastic swell and rupture.
In another coin type (button type) non-aqueous electrolyte secondary battery using the 3 V-class lithium-containing manganese oxide Li4Mn5O12 for the positive electrode and carbon, with which lithium is brought into contact or in to which lithium is doped electrochemically, for the negative electrode, too, the electrolyte and the negative plated doped with lithium react with reach other and invite drastic swell and rupture.
In the conventional coin type (button type) non-aqueous electrolyte secondary battery, all of the electrolyte, the separator and the gasket cannot withstand the reflow temperature. Therefore, boiling and fusion develop.
Even when the material is changed to a heat-resistant material for each of the electrodes and other members, a battery capable of coping with the reflow temperature cannot be fabricated. Though the problem of swell and rupture at the reflow temperature can be solved, another problem occurs that the battery characteristics get extremely deteriorated.
For, even though the material may seem stable, the combination of such materials is not so selected as to achieve the highest stability and performance.
In order to solve the problems described above, it is an object of the present invention to provide a battery capable of exhibiting satisfactory characteristics even after reflow soldering, by employing all of the following means for improvement.
(1) Heat-resistant Electrodes and Members are Selected.
The battery of the present invention uses a positive electrode active material formed of LiCoO2, LiNiO2 or LiMn2O4, each being an oxide containing mobile lithium. Anatase type titanium oxide (TiO2), lithium titanate having a spinel structure or a molybdenum oxide is used for the negative electrode.
LiCoO2, LiNiO2 and LiMnO4 as the oxide containing mobile lithium, anatase type titanium oxide and spinel type lithium titanate are difficult to invite a drastic reaction at the reflow temperature. Therefore, in order to obtain a battery that allows reflow soldering, heat-resistant materials are used for the electrolyte, the separator and the gasket as the constituent elements of the battery.
(2) Heat Stability of the Electrode Active Materials is Further Improved.
As a result of various studies conducted so as to improve stability of the electrode active materials, it has been found that the highest stability can be achieved at the reflow temperature by reducing the quantity of dust.
(3) Degradation of Battery Characteristics is Restricted by Heat-treatment of the Electrodes.
Electrodes comprising the electrode active material, a conduction adjuvant and an organic binder are heat-treated at 200 to 450xc2x0 C. Consequently, the active portions of the active material, organic binder and conduction adjuvant (dust, non-reacted portions during synthesis, etc.) are rendered inactive, and wettability between the electrolyte and the electrode is improved. Because the electrolyte having particularly high stability has a high viscosity and does not easily permeate into the electrode, the combined effect with the heat-treatment is great.
The optimum combination of these technical means gives the coin type (button type) non-aqueous electrolyte secondary battery capable of withstanding the reflow temperature.
(4) The Best Combination is Searched in the Selection of the Electrolyte and the Heat-resistant Members.